峡区晚奥陶世地球化学异常与奥陶系-志留系之交环境变迁

LATE ORDOVICIAN CHEMICAL ANOMALY AND THE ENVIRONMENTAL CHANGES ACROSS THE ORDOVICIAN-SILURIAN BOUNDARY IN YANGTZE GORGES

通过对峡东地区宜昌王家湾、黄花场和分乡等三条重要奥陶系 -志留系界线剖面碳同位素 (δ1 3C)地球化学异常对比研究 ,以及远安苟家垭和秭归新滩奥陶系 -志留系界线剖面稀土元素分布型式、δCe异常的分析 ,和对苟家垭奥陶系 -志留系界线剖面微量元素 (Rb、Sr、Sc、V、Ni等 )地球化学异常的研究 ,揭示了奥陶系 -志留系界线附近地球环境的巨大变化 ,论述了环境变化对生物界的影响 ,将奥陶系五峰组划分为 2个三级层序和 5个四级层序 ,为奥陶系

Based on the study of carbon isotopic anomaly of three important Ordovician Silurian boundary sections in the east of the Yangtze Gorges area, the analysis of the REE distribution pattern and Ce anomaly of Ordovician Silurian boundary sections at Goujiaya of Yuan′an and Xintan of Zigui, and the study of some significant trace elements in the Goujiaya section, the authors discussed the environmental changes during the period. The δ 13 C curve, in one hand, demonstrated that its deviation coincided with the appearance of Manosia fauna and the blooming of Hirnantia Kinnella fauna, in other words, the decline and mass extinction of graptolite fauna. On the other hand, it shows different deviations responding to transgression and regression, respectively, i.e.positive deviation at the beginning of regression induced by the forming of Gondwana ice caps, negative deviation at the beginning of transgression induced by the rapid melting of the ice caps on Gondwana. A similar phenomenon was shown in the Ce anomaly curve, i.e.,the increase of negative anomaly of Ce responding to transgression and the incease of positive anomaly of Ce connecting to regression. According to the Ce anomaly curve, 5 regression/transgression couples are recognized in Goujiaya section. Based on integrated analysis, this paper puts forward that the Wufeng Fm.could be divided into two third grade sequences and five forth grade sequences. Remarkable deviation of trace elements are also recorded at the base of the persculptus Zone at Goujiaya section, which revealed the significant changes of global environment across the Ordovician Silurian boundary.